The lipids are one of four major families of biochemical compounds, the other three being carbohydrates, proteins, and nucleic acids. Biochemical compounds are organic (carbon-containing) compounds that occur in living organisms. The lipids are unique among organic and biochemical families because of the way in which they are classified. In all other families, members are categorized because of similar chemical structure and similar chemical properties. Lipids are characterized instead on the basis of a single physical property, their solubility. Lipids tend to be insoluble in water, but soluble in certain organic solvents such as benzene, chloroform and ether. The lipid family contains, therefore, a rather wide range of compounds that are structurally quite different from each other. The members of the family are often divided into four subcategories: (1) the simple lipids, (2) the compound lipids, (3) the steroids, and (4) the terpenes.
The simple lipids include waxes, fats, and oils. These compounds are structurally similar to each other because they consist of alcohols combined with long organic acids known as fatty acids. Waxes are constructed of a single molecule each of alcohol and acid while fats and oils contain three fatty acid molecules for each alcohol molecule. Fats are distinguished from oils in that the former are solids and the latter, liquids. As common, naturally occurring substances, the simple lipids were one of the first group of organic compounds studied by chemists. The French chemist Michel Eugène Chevreul showed in 1816, for example, that during the manufacture of soap, a portion of the fat molecule (the fatty acid) combines with lye to make the final product (the soap).
Compound lipids consist of a simple lipid and some other group, such as a phosphoric acid fragment or a nitrogen-containing alcohol. Members of this family have many important functions in biological systems. For example, one group of compound lipids, the glycolipids, occur in the membranes of brain and nerve cells. Because the compound lipids are so complex and because they occur in such small concentrations, little has been known about them until recently. One of the best known compound lipids, lecithin, was discovered, however, as far back as the 1870s by the German biochemist Ernst Hoppe-Selyer.
The steroids are a fascinating group of compounds that includes such diverse representatives as cholic acid, a component of bile; estrone, progesterone, testosterone, and other sex hormones; vitamin D; cortisone; and cholesterol. Scientific interest in the steroids dates back to the ancient Greeks who viewed bile as one of the four humours that determined one's health. The modern study of the steroids received its impetus, however, from the work of Heinrich Otto Wieland (1877-1957) and Adolf Windaus (1876-1959), German chemists and personal friends. The two began their studies of the bile acids prior to World War I. Independently, they extracted cholanic acid from bile. They also showed how a number of apparently dissimilar substances, including the bile acids, cholesterol, vitamin D, and the gonadotrophic hormones all share a common chemical structure. Both Windaus and Wieland attempted to determine the precise structure of the complex, four-ringed configuration of cholesterol and its steroid cousins. Although neither was entirely successful in this endeavor, their contributions to the study of steroids was recognized by the awarding of the Nobel Prize for their pioneering work in 1927 (to Wieland) and 1928 (Windaus).
The terpenes are another very large class of naturally-occurring compounds, many with characteristic and pleasing aromas. The oils of camphor, menthol, lemon, orange, basil and geranium are examples of terpenes, as is natural rubber. The German chemist Otto Wallach is often called the father of terpene chemistry because of his long and exhaustive study of these compounds stretching from the 1880s through the 1910s.
Recent research on lipids has focused on the lipoproteins. Lipoprotiens are organic compounds composed of both protein and a lipid. There are at least four groups of lipoproteins present in plasma: High-density lipoproteins (HDL), low-density lipoproteins (LDL), very low density lipoproteins (VLDL), and chlyomicrons. The different densities refer to the relative amounts of lipid and protein. The higher the density, the higher the protein to lipid ratio. LDLs transport cholesterol to cells and deposit excess cholesterol in the blood vessels, which increases the risk of arteriosclerosis. HDLs, however, transport cholesterol from the tissues to the liver where it is excreted, lowering the risk of arteriosclerosis. A high HDL to total cholesterol ratio is the best indication of decreased risk of arteriosclerosis. HDL levels vary from person to person and can be influenced by such things as heredity, sex, age, and physical activity. Smoking and obesity have been shown to decrease plasma HDL levels.
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